Hybrid vehicles and electric vehicles are now attracting considerable attention as automobiles taking into account environmental matters. Some hybrid vehicles are now commercially available.
Such hybrid vehicles employ a DC power supply, an inverter, and a motor driven by the inverter as well as a conventional engine, as the power source. Specifically, power is generated by driving the engine, as well as by the rotation of the motor based on converted alternating voltage, achieved by conversion of a direct current voltage from a DC power supply by means of an inverter. An electric vehicle employs a DC power supply, an inverter, and a motor driven by the inverter as the power source.
In such hybrid vehicles and electric vehicles, an approach is known to boost a direct current voltage from a DC power supply by a voltage-up converter, and supplying the boosted direct current voltage to the inverter that drives the motor (for example, Japanese Patent Laying-Open No. 2001-275367).
Specifically, a hybrid vehicle or an electric vehicle incorporates a motor driver shown in FIG. 23. Referring to FIG. 23, a motor driver 300 includes a DC power supply B, system relays SR1 and SR2, capacitors C1 and C2, a bidirectional converter 310, a voltage sensor 320, and an inverter 330.
DC power supply B outputs a direct current voltage. System relays SR1 and SR2 supply the direct current voltage from DC power supply B to capacitor C1 when turned on by a control device (not shown). Capacitor C1 smoothes the direct current voltage supplied from DC power supply B via system relays SR1 and SR2, and supplies the smoothed direct current voltage to bidirectional converter 310.
Bidirectional converter 310 includes a reactor 311, NPN transistors 312 and 313, and diodes 314 and 315. Reactor 311 has one end connected to a power supply line of DC power supply B and its other end connected at an intermediate point between NPN transistors 312 and NPN transistors 313, i.e. between the emitter of NPN transistor 312 and the collector of NPN transistor 313. NPN transistors 312 and 313 are connected in series between the power supply line and the ground line. The collector of NPN transistor 312 is connected to the power supply line. The emitter of NPN transistor 313 is connected to the ground line. Further, diodes 314 and 315 conducting a current from the emitter side to the collector side are connected between the collectors and emitters of NPN transistors 312 and 313, respectively.
Bidirectional converter 310 has NPN transistors 312 and 313 turned on/off by a control device (not shown) to boost the direct current voltage from capacitor C1 and provide the output voltage to capacitor C2. When the hybrid vehicle or electric vehicle in which motor driver 300 is incorporated is under regenerative braking, bidirectional converters 310 is powered by alternating current motor M1 to down-convert the direct current voltage converted by inverter 330 and supply the down-converted voltage to capacitor C1.
Capacitor C2 smoothes the direct current voltage from bidirectional converter 310 to provide the smoothed direct current voltage to inverter 330. Voltage sensor 320 detects the voltage across capacitor C2, i.e. the output voltage Vm of bidirectional converter 310.
When direct current voltage is supplied from capacitor C2, inverter 330 converts the direct current voltage into alternating voltage under control of a control device (not shown) to drive alternating current motor M1. Accordingly, alternating current motor M1 is driven to generate the torque specified by a torque control value. When the hybrid vehicle or electric vehicle in which motor driver 300 is incorporated is under regenerative braking, inverter 330 converts the alternating voltage generated from alternating current motor M1 into a direct current voltage under control of the control device to supply the converted direct current voltage to bidirectional converter 310 via capacitor C2.
When the direct current voltage output from DC power supply B is boosted and the output voltage Vm is to be provided to inverter 330 in motor driver 300, feedback control is effected so that output voltage Vm detected by voltage sensor 320 is equal to a voltage control value Vdccom. This feedback control is PI control. The PI control gain is determined so that output voltage Vm is equal to voltage control value Vdccom.
In such a conventional motor driver, the PI control gain is determined and feedback control is effected employing the determined PI control gain to set the boosted output voltage Vm equal to voltage control value Vdccom.
When the PI control gain is determined under a certain condition and control is fixed to the determined PI control gain, any change in output voltage Vm and voltage control value Vdccom will cause variation in the adjustment of the voltage applied across NPN transistor 313 in accordance with output voltage Vm even if the difference between output voltage Vm and voltage control value Vdccom is constant. As a result, the problem of variation in the follow-up property of output voltage Vm (e.g., a transient response property) with respect to voltage control value Vdccom will occur.